1. Technical Field
The present invention relates to a vaporizer and a vaporizing method, which is preferably used for a deposition device such as a MOCVD, as well as a other various devices.
2. Background Art
The development of DRAM causes a problem concerning memory capacitance depending on microfabrication. Some measures for the problem about the memory capacitance is necessary because the degree of performance of the capacitance is required to be the same as the former generation in view of potential software errors. As the measure for the above-described problem, a capacitor area has been increased as follows. A planar structure has been taken as the cell structure up to 1M capacity. However, the three-dimensional structure, called a stack structure or a trench structure, has been taken as the cell structure for 4M or above. Moreover, as a dielectric film, a film which laminates an oxidation film from the heat oxidation film of a Si substrate and a CVD nitride film (the laminated film is called an ON film in general) on poly Si is adopted. The film types utilizing the back side of a plate, a cubic type using the side in stack type, were taken in order to increase area to contribute to capacity 16M DRAM.
However, deterioration of yield by an increase of process number by a complicated process and increase of difference in steps is brought into question by such a three-dimensional structure, and it is assumed that realization after the 256 M bit limit is difficult. Therefore, as one way to further increase a degree of integration without changing the structure of the present DRAM, a method to shift a dielectric for a capacitance to the one having a higher dielectric constant is devised. Thus, at first, thin films of paraelectric oxides of single metals with high dielectric constant, such as Ta2O5, Y2O3, and HfO2, attract attention. As for each ratio dielectric constant, Ta2O5 is 28, Y2O3 is 16, HfO2 is around 24, and is 4 to 7 times of SiO2.
However, in an application after 256M DRAM, a three-dimensional capacitor structure is required. (BaxSr1-x)TiO3, Pb(ZryTi1-y)O3, (PbaL1-a)(ZrbTi1-b)O3 are regarded as promising materials which have the higher relative dielectric constant than the above-described oxides and can be expected to apply to the DRAM.
Moreover, recently, a layered structure of Bi group which has a very similar crystal structure to the one of a superconductive material greatly attracts the attention because it has a high dielectric constant, has a self polarization characteristic of a ferroelectric, and is superior as a nonvolatile memory. Generally, the thin film formation of SrBi2TaO9 ferroelectric is done by MOCVD (metalorganic chemical vapor deposition) method—the practical and promising method. The raw material of the ferroelectric thin film includes, for instance, three kinds of organometallic complex, Sr(DPM)2, Bi(C6H5)3, and Ta(OC2H5)5, melt respectively in THF (Tetorahidorofran), hexane, and other solvents, and this combination is used as a raw material solution. Sr(Ta(OEt)6)2 and Bi(OtAm)3 can be melted in hexane and other solvents and used as a raw material solution. Note that DPM is an abbreviation of bis(diphenylphosphino) methane.
Table 1 shows each material characteristic.
TABLE 1Characteristics of a raw material for aferroelectric thin filmBoiling point (° C.) andMeltingpressure (mm Hg)point (° C.)Sr(DPM)2231/0.1210Bi(C6H5)3130/0.180Ta(OC2H5)5118/0.122THF67−109Sr(Ta(OEt)6)2176/0.1130Bi(OtAm)3 87/0.190
A device to be used for the MOCVD method includes a reaction part in which the SrBi2TaO9 thin film material is deposited after a phase reaction and a surface reaction and further includes a supply part which supplies the SrBi2TaO9 thin film material and an oxidizer to the reaction part. Moreover, the supply part is provided with a vaporizer for vaporizing the thin film material.
Respective methods shown in FIG. 16 are known, so far, as a technology concerning the vaporizer. The one shown in FIG. 16(a) is called a metal filter type, and it is a method that introduces and vaporizes the raw material solution heated to a prescribed temperature into a metal filter used to increase the contact area between an atmospheric gas and the raw material solution of the SrBi2TaO9 ferroelectric thin film.
However, in this technology, the metal filter can become clogged by the effect of several hours of vaporization. As such, there is a problem that it cannot bear the long-term use. The present inventor guessed that it is because the solution is heated and vaporized from a solution having a low vaporization temperature.
FIG. 16(b) shows a technology that the raw material solution is discharged from a small hole of 10 μm by applying the pressure of 30 kgf/cm2 to it and vaporized by the expansion.
However, the problem of clogging the small hole by the use for several hours, and not enduring a long-term use is also observed in this technology.
Moreover, the problem that a steady raw material supply cannot be achieved to a reaction is created because the solvent with the highest vapor pressure (in the following case, THF) evaporates promptly and an organometallic complex deposits and adheres on the heating face, when the raw material solution is a mixture solution of organometallic complexes, for instance, Sr(DPM)2/THF, Bi(C6H5)3/THF and Ta(OC2H5)5/THF, and this mixture solution is evaporated by heating. As for all these methods shown in FIG. 1, the energy (e.g., the number of calories) which could vaporize or alter the solvent in the liquid or mist state turns up to be added.
In addition, in MOCVD, to obtain an excellent film uniformity, a vaporization gas in which the raw material solution is dispersed uniformly is required to be obtained. However, in the prior art, such a requisite is not necessarily complied with.